Revert "Switch libraries to using new tags."

sdk/lib/isolate/timer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of dart.isolate;
-
 abstract class Timer {
   /**
    * Creates a new timer. The [callback] callback is invoked after
    * [milliSeconds] milliseconds.
    */
   external factory Timer(int milliSeconds, void callback(Timer timer));
 
   /**
    * Creates a new repeating timer. The [callback] is invoked every
    * [milliSeconds] millisecond until cancelled.
    */
   external factory Timer.repeating(int milliSeconds,
                                    void callback(Timer timer));
 
   /**
    * Cancels the timer.
    */
   void cancel();
 }